Atomic clocks using a hydrogen maser (a device for Microwave Amplification by Stimulated Emission of Radiation) are well known in the art. The microwave radiation of the hydrogen atoms in the maser cavity is coherent and, if losses in the cavity are sufficiently low, self-sustained oscillation can be obtained if the resonant frequency of the cavity is substantially equal to the frequency of the atomic radiation, namely 1420.405751 MHz. Such a maser cavity is typically 28 centimeters in diameter and 28 centimeters in length. A clock signal may be generated by referencing the phase of a voltage controlled oscillator to the microwave radiation of the maser in a feedback control loop. A major disadvantage of such an atomic clock is that it is bulky due to the large (28 centimeters) size of the maser cavity.
If the size of the maser cavity is reduced in an effort to miniaturize the atomic clock, the losses in the cavity will increase to the extent that self-sustained oscillation cannot be achieved. Therefore, external radiation stimulation is required to obtain the atomic resonance signal. However, the frequency and amplitude of the external stimulation have to be precisely controlled to prevent distortions of the atomic resonance signal and consequent frequency shifts of the clock output. One solution to this problem is described in U.S. Application, Ser. No. 043,075 filed May 29, 1979 by the Applicant of this application, entitled "Atomic Frequency Standard Using Free Induction Technique", and assigned to the assignee of this application. One disadvantage of the free induction technique of the above-referenced patent application is that measurements of the maser radiation are performed by the clock electronics periodically instead of continuously so that the signal-to-noise ratio of the referenced maser radiation is limited by the duty cycle of the measurements. Therefore, it has been a long standing problem in the art that miniaturization of a maser atomic clock requires external stimulation which either introduces signal distortion, or, as in the above-referenced patent application, reduces the duty cycle of the clock electronics, thereby decreasing the signal-to-noise ratio of the system.
Another problem area in maser atomic clocks is the limited stability of the resonant frequency of the microwave cavity. Since the cavity and atoms form a coupled pair of oscillators, changes in cavity resonant frequency will produce systematic variations in the measured atom transition frequency and, hence, in the output of the frequency standard. Previous maser cavity designs relied on thermal and mechanical means for cavity frequency stabilization. These designs are susceptible to long term drift due to material creep and thermal control system variations.
Therefore, it has been a desirable goal in the art to provide a maser having a cavity with a stable resonance frequency and of reduced size but in which the atomic oscillation is self-sustained and does not have to be stimulated by an external source.